The use of fiber optic elements for sensing a variety of parameters is known. The typical element is the fiber which transmits light from a source to a transducer and then to a detector. The transducer alters a characteristic of the light transmitted by the fiber, usually intensity, and the detector produces a signal which is a measure of a selected parameter to which the transducer is sensitive.
Fiber optic sensors have many characteristics, such as size, weight, electromagnetic isolation, safety, and ease of installation which make them desireable for a variety of uses, such as for a monitoring and control system on a ship.
In a typical system, the input light is modulated, and the sensor filters out the detected signal at the frequency of modulation. This system eliminates the effects of intensity fluctuations in the source. Another technique is to provide two sources at different wavelengths and which are modulated at different frequencies. According to this technique, one of the wavelengths is chosen such that it is affected by the transducer to measure the parameter of choice. The second wavelength is selected so that it is not affected by the transducer but is affected by other factors in the system in the same fashion as the first wavelength. Thus, the second wavelength acts as a reference to indicate all variations in the intensity of the light which are not related to the parameter being measured. The ratio of the intensities of the two wavelengths can be obtained by filtering the output signals produced by the detector at the two modulating frequencies and by determining the ratio of the measured intensities as a measure of the selected parameter.
While basic fiber optic measuring systems have been used in a variety of situations, they have not generally been used in a network which provides great facility in substitution of light sources, transducers, or detectors, or which provides the redundancy necessary for safety.